Recently, color-copying methods have come into practical use, in which an electrostatic latent image carrier is exposed to spectrally separated light to form an electrostatic latent image of an original and the latent image is further developed with color toners to obtain a colored copy image or the respective color copy images are superimposed to obtain a full color copy image. As toner used in the foregoing are manufactured color toners of yellow, magenta, cyan and the like which are formed by dispersing a dye and/or pigment of the respective colors in a binder resin.
The foregoing electrophotographic process, in general, forms images in accordance with the following steps. First, exposing light information in response to image information onto an electrostatic latent image carrier (hereinafter, also denoted as a photoreceptor) composed of a photoconductive material, through various methods forms a latent image on the photoreceptor. The electrostatic latent image formed on the photoreceptor is developed with a charged toner to form a toner image. The formed toner image is transferred onto an image recording medium (hereinafter, also denoted as transfer material) and fixed onto the transfer material using a thermal fixing apparatus.
In the foregoing color image forming method employing the electrophotographic process, electrostatic latent images formed on the photoreceptor correspond to image information which have been separated to the respective colors of yellow, magenta, cyan and black and developed with a toner having a color identical to the respective image information. The development step is repeated four times for the respective colors to form a full color image.
Organic pigments and dyes known in the art have conventionally been used as a colorant used for electrophotographic toner but they exhibit various defects. For instance, organic pigments are generally superior in heat resistance or lightfastness, compared to dyes but existing in toner particles in the form of granular dispersion results in enhanced masking, leading to reduced transparency. In general, low dispersibility of pigments results in deteriorated transparency and lowered chromaticness, deteriorating color reproducibility of images.
When different color toners are superimposed, transparency of fixed toners is needed to visually confirm color of a toner existing in the lowest layer. Dispersibility or coloring power of a colorant become necessary to maintain color reproducibility of an original.
To overcome the foregoing defects of pigments, there were proposed a technique for enhancing transparency in which application of a flushing process as a means for dispersing pigments achieved a pigment dispersion diameter in the order of sub-microns, formed of primary particles without forming aggregated secondary particles; and a technique for improving electrification property, fixability and image uniformity by covering pigment particles with a binding resin or a shelling resin, as described, for example, in JP-A Nos. 9-26673 and 11-160914 (hereinafter, the term, JP-A refers to Japanese Patent Application Publication).
However, even when forming toner images by using the thus proposed toner, it is difficult to achieve sufficient transparency, specifically in the case of a pigment toner.
In color imaging apparatuses, all of color reproduction can be achieved, in principle, by a subtractive color system using the three primary colors of yellow, magenta and cyan. In practice, however, the spectral property provided when dispersing a pigment in thermo-plastic resin or the color-mixing characteristic provided when superimposing different toners results in a reduced range of color reproducibility or lowered chromaticness so that problems to be overcome still remain to achieve faithful color reproduction of an original.
There were also introduced toners using dyes and toners using a mixture of a dye and a pigment. In a toner using a dye, the dye existed in the form of being dissolved in a binding binder so that superior transparency and chromaticness were achieved but inferior lightfastness or heat resistance resulted as compared to pigments, as described, for example, in JP-A Nos. 5-11504 and 5-34980.
With respect to heat resistance, in addition to lowered density due to decomposition of a dye, there were produced problems that when fixing a toner image by a heated roller, the dye was sublimed, easily causing in-machine staining and it was also dissolved in silicone oil used in the fixing stage and finally adhered to the heated roller, causing the so-called off-set phenomenon. To overcome such defects of dyes, there were proposed a technique of using specific anthraquinone type dyes as a magenta toner to improve lightfastness and sublimation ability in compatible with color reproduction and an encapsulated toner in which a core containing polymer resin and a color dye was covered with a polymer, as described, for example, in JP-A Nos. 5-72792 and 8-69128.
However, even when forming toner images using electrophotographic toners, as proposed above, it was difficult to achieve sufficient heat resistance (sublimation resistance) and lightfastness.